If i could only advice one external controller for the Doepfer A-100 system it would definitely be the Doepfer MAQ16/3 MIDI/Analog Sequencer.
This versatile sequencer has 3 rows of 16 knobs, and outputs a CV (for pitch) and a Gate (trigger) signal for each row. It also has a MIDI input for synchronizing, and a MIDI output to connect any sound generating MIDI device.
Different running modes are available for each row, such as Forward/ Backward/ Random/ Pendulum and many more, so the possibillities are almost limitless. Other parameters that can be altered in the sequencer are First/Last step, Gate time, Tempo, and almost any other MIDI event type.
My only personal negative point that i would like to mention is the limit of 30 sequence memory spaces, way too little in my opinion, and i know it is more than what sequencers could store long time ago, but hey... can't this be upgraded to 99 memories ( please!? )
There are many alternatives for the MAQ16/3, the A-155 Analog/Trigger Sequencer is Doepfers own dedicated in-system sequencer module that can be extended with the A-154 Enhanced Sequencer Controller, but still doesn't have the full funcionality of the MAQ.
(Analog) sequencers of other brands are also quite popular, some in a eurorack module form-factor, others in the shape of desktop controllers...
What's your favorite? Feel free to leave a comment below!